There is a wide-ranging demand for increased communications capabilities, including more channels and greater bandwidth per channel. The needs range from long distance applications such as telecommunications between two cities to extremely short range applications such as the data-communications between two functional blocks (fubs) in a semiconductor circuit with spacing on the order of a hundred microns.
Optical fibers can carry information encoded as optical pulses over long distances. The advantages of optical media include vastly increased data rates, lower transmission losses, lower basic cost of materials, smaller cable sizes, and almost complete immunity from stray electrical fields. Other applications for optical fibers include guiding light to awkward places (e.g., surgical applications), image guiding for remote viewing, and various sensing applications.
Optical fibers or waveguides provide an economical and higher bandwidth alternative to electrical conductors for communications. A typical optical fiber includes a silica core, a silica cladding, and a protective coating. The index of refraction of the core is higher than the index of refraction of the cladding to promote internal reflection of light propagating down the silica core.
Waveguides have been developed comprising a mixture of silica (SiO2) and silicon nitride (Si3N4), often referred to as SiON. The indexes of refraction of the core and cladding can be controlled by controlling the nitrogen content. That is, the nitrogen content of the core will be higher than that of the cladding to give the core a suitably higher index of refraction than the cladding.
However, the differences in the index of refraction of the core and cladding also result in birefringence, or the separation of the light pulse or ray into two unequally refracted pulses or rays. As a result, part of the light transmission is lost. For fiber optic communication systems where long range fiber optic communication is utilized, there is a need for optical and electro-optic devices that are substantially free from birefringence.
In general, birefringence is the difference between a refractive index nTM for the TM mode having a field component perpendicular to the substrate and a refractive index nTE for the TE mode having a field component parallel to the substrate, or, the birefringence equals nTM-nTE.
The majority of fiber optic telecommunications systems use standard single-mode silica fiber that does not preserve the polarization of the transmitted light. For such systems, the polarization state of the light signal in the optical fiber at any point and at any time is unknown and subject to variation over time and distance as a result of environmental and other changes that occur along the transmission path of the signal. If devices placed at any point in the fiber transmission path or at its end have response characteristics that depend on the polarization state of the light (i.e., polarization dependence), the signal may be degraded or lost altogether.
As integrated optical and electro-optical devices are employed in fiber optic systems for which the polarization state of the light signal is unknown, a need arises to circumvent or minimize the consequences of the polarization dependence and birefringence of these devices.
The most popular approach for reducing the effects of birefringence has been to introduce additional components to control the state of polarization of the light signal before its introduction to the polarization-sensitive device.
A more satisfactory approach would be to provide a waveguide device with a small polarization dependence and birefringence thereby causing only negligible transmission degradation.